Rheology - Battery Design

Rheology is the study of the flow and deformation of materials when they are subjected to applied stress or forces.

In battery manufacturing understanding rheology is crucial because it dictates how a material will behave when mixed, pumped, coated, and stored. Reynolds et al [1] summarise the importance:

understanding the rheological properties and slurry microstructure is crucial to optimizing the manufacturing process of battery electrodes. The rheological properties provide valuable insights into the effectiveness of mixing and the internal structure of the slurry, including agglomeration, adsorption of components at interfaces, and the formation of particle-polymer networks. These properties play a vital role in determining the performance of the slurry in subsequent manufacturing steps such as coating and drying.
Carl D. Reynolds, Helen Walker, Ameir Mahgoub, Ebenezer Adebayo and Emma Kendrick, Battery electrode slurry rheology and its impact on manufacturing, Energy Adv., 2025, 4, 84-93

Key rheological parameters used to characterize and optimize battery slurries:

Viscosity and Shear-Thinning Behaviour Viscosity measures a material’s resistance to flow. Battery slurries are typically non-Newtonian fluids that exhibit shear-thinning behaviour, meaning their apparent viscosity decreases as the applied shear rate increases.
- Low-shear viscosity (Zero-shear viscosity): At rest or low shear rates, the slurry must maintain a high viscosity to prevent heavy solid particles (like active materials) from settling or separating during storage.
- High-shear viscosity (Infinite-shear viscosity): During pumping and slot-die coating, the slurry experiences high shear forces. A lower viscosity under these conditions is necessary to allow the slurry to flow smoothly, keep pressure drops manageable, and form a thin, uniform coating.
Yield Stress (σy) Yield stress is the minimum amount of stress required to initiate significant flow in the slurry. Below this stress threshold, the slurry behaves like a viscoelastic solid; above it, it flows like a liquid. A sufficient yield stress is vital because it maintains the internal network of the slurry (often formed by conductive carbon black), preventing the sedimentation of particles while at rest and ensuring the wet coating does not slump, sag, or run after being applied to the current collector.
Viscoelasticity (Storage and Loss Moduli) Viscoelasticity describes a material’s combination of solid-like (elastic) and liquid-like (viscous) characteristics. This is typically measured using small-amplitude oscillatory shear tests to find two components:
- Storage Modulus (G′): Represents the elastic, solid-like behavior of the slurry.
- Loss Modulus (G′′): Represents the viscous, liquid-like behaviour of the slurry. Ideally, at low frequencies (at rest), the slurry should exhibit a “gel-like” structure where the storage modulus is greater than the loss modulus (G′>G′′) to stabilize against sedimentation. Conversely, at high frequencies (during application), the slurry should be liquid-like (G′′>G′) for easy and uniform coating.
Thixotropy and Structural Recovery Time Thixotropy is a time-dependent property where a material’s viscosity decreases under constant shear and then gradually recovers once the shear is removed. Thixotropic recovery is heavily monitored because it dictates how the slurry behaves during the “levelling” period between exiting the coating head and entering the drying oven. If the slurry recovers its structure too quickly, it may trap air or fail to level out; if it recovers too slowly, it can cause pooling, phase separation, or drying-induced cracks.
Extensional Viscosity While shear viscosity measures resistance to sliding flows, extensional viscosity measures resistance to stretching flows. The mixing and coating steps (especially slot-die coating) contain significant extensional flows. Measuring this parameter helps engineers predict flow uniformity and avoid coating defects that shear rheology alone cannot forecast.